Ultrasound imaging is used in the medical field to image inside a patient. Ultrasound imaging may be performed with an internal ultrasound imager received within the patient (e.g., intravascular ultrasound system) or an external ultrasound imager placed on the patient's skin. An ultrasound imager comprises one or more ultrasound transducers that emit ultrasonic waves into the patient. The ultrasound waves are reflected back to the transducer by layers of tissue or other structures in the patient as echo waves. A transducer converts received echo waves into electrical signals that are representative of the strength of the echo waves. The electrical signals are processed by an ultrasound image processor into an ultrasound image.
Current ultrasound transducers are coherent sensing devices that suffer from an imaging phenomenon known as speckle noise. Speckle noise is caused by the interference, both constructive and destructive, of echo wave fronts originating from scattering sources that are too small to be resolved by the transducer. A simple example of this is illustrated in FIGS. 1a and 1b. FIG. 1a shows an ultrasound wave 112 that is emitted from a transducer 110 and propagates towards a structure in the body, e.g., blood vessel wall 115. The ultrasound wave 112 is typically a periodic pressure wave having a frequency in the megahertz range. FIG. 1b shows an example of echo wave fronts 122a and 122b originating from two scattering sources 120a and 120b that are too small to be resolved by the transducer 110. The transducer 110 coherently sums the echo wave fronts 122a and 122b impinging on the active aperture of the transducer 110, and produces an electrical signal based on the coherent sum. If the echo wave front 122a impinges the transducer 110 at its peak and the echo wave front 122b impinges the transducer 110 at its trough, then the echo wave fronts 122a and 122b destructively interfere, and tend to cancel each other out. This destructive interference results in a dark spot in the ultrasound image at the blood vessel wall 115 even though the blood vessel wall 115 should appear bright in the image. Typically, scattering sources are distributed throughout the area of the body being imaged. The destructive interference of echo wave fronts originating from these scattering sources give rise to dark spots in the image that characterize speckle noise.
Speckle noise reduces the quality of the ultrasound image and, therefore, the ability of physicians and computers to perform diagnoses based on the image. Speckle noise is one reason many image processing algorithms, that work reasonably well for other imaging modalities such as CT or Magnetic Resonance (MR), do not work well for ultrasound images.
Therefore, there is a need in the art to suppress speckle noise in ultrasound imaging.